1. Field of the Invention
The present invention relates to a specific phase change ink composition that is useful for printing on various substrates, particularly transparencies, to produce high quality medical images such as x-ray, ultrasound, nuclear medicine, magnetic resonance, computer tomography, positron emission tomography, and angiography. In particular, the present invention relates to a phase change ink which may be used to create images that match the print quality and perceived appearance of black silver halide medical images. The ink is formulated by combining a black colorant, preferably a dye, having a deficient low absorbance region in the human visible response spectrum with at least one other colorant, preferably a dye, having a high absorbance region corresponding to the deficient low absorbance region of the black dye.
2. Background of the Invention
X-ray photographic films have long been used in medical imaging to help perform medical diagnoses. The black color generated from the development of silver halide photographic film has a somewhat brownish-black hue. A majority of the medical imaging photographic films today are produced on a blue tinted polyester transparent support to produce a more neutral black shade upon development of the silver halide film. Radiologists have been trained to read these films by looking for subtle gradations in optical densities in this particular black shade. Silver halide films for medical imaging have proven extremely useful in making medical diagnoses. However, the use of silver halide compositions for medical imaging has several disadvantages. These unimaged films are expensive, have limited shelf life, must be protected from light and other radiation, require chemical wet processing to develop an image, and disposal of the development chemicals employed raise environmental concerns.
It has been proposed to use in an ink jet imaging system commercially available phase change ink compositions having a black dye colorant on non-silver halide x-ray substrates having the same blue-tint or a clear or opaque appearance in medical diagnostic imaging applications.
It has been found that the resulting non-silver halide-containing films imaged with phase change ink in an ink jet imaging system are of diagnostic quality; however, the black color is different than what radiologists are accustomed to viewing. It was determined this different color (i.e. a bluish-black shade on the standard blue-tinted x-ray substrate) is due to great differences in the absorbance of the black dye over the visible wavelength spectrum from about 380 nanometers (nm) to about 670 nm compared to the absorbance of images generated on silver halide containing film. For example, with a standard black dye, such as Color Index (C.I.) Solvent Black 45, there are high and low absorbance regions. Furthermore, the absorbance in the 475 nm region was only about 60-70 percent of the absorbance in the 580 nm region. Accordingly, more light passes through the imaged substrate in the low absorbance region than in the higher absorbance region, causing an unintended shading difference on the imaged film substrate, again compared to an image created on a silver halide film.
Therefore, there is a need for an improved phase change ink composition which overcomes this problem created by the attempted use of commercially available back phase change ink for use on non-silver halide photographic film in medical diagnostic imaging applications.